Techniques for performing spectroscopy are known. In many of these techniques, light from a broadband source, (e.g., a tungsten filament lamp), impinges upon a sample and is then separated into spectral components via a prism or grating. The prism or grating is rotated to bring each spectral component in turn into a detector where a measurement is performed. In other known spectroscopy techniques, tunable narrowband light sources (e.g. lasers) are directed at a sample, and the transmitted light is detected.
While traditional absorption methods in the near and mid-infrared regions may be performed, such techniques suffer from poor sensitivity due to low resolution and low sensitivities of available detectors. Far-infrared (far-IR) and terahertz (THz) spectral regions are not easily accessible using current technology, yet these bands are rich in spectroscopic information. Another disadvantage of existing techniques is the requirement for cumbersome and frequent calibration. Further, in conventional spectroscopy, noise from background radiation degrades both resolution and signal-to-noise ratio (SNR).
Photons are quanta of electromagnetic energy. Multiple photons may be random or entangled. Random photons are not entangled together and exist as independent entities. In contrast, entangled photons have a connection between their respective properties.
Two photons entangled together are referred to as an entangled-photon pair (also, “biphotons”). Traditionally, photons comprising an entangled-photon pair are called “signal” and “idler” photons. Measuring properties of one photon of an entangled-photon pair determines properties of the other photon, even if the two entangled photons are separated by a distance. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of an entangled-photon pair cannot be factored into a product of two individual quantum states.
Photon properties that may be entangled include time, frequency, and angular momentum. Photons that are entangled in time are generated nearly simultaneously. For given optical path lengths traveled by constituent photons in a temporally-entangled-photon pair, detecting one of the photons places limits on the times at which the other photon may be detected. Photons that are entangled together in frequency have their frequencies related, in that measuring the frequency of one such photon determines the frequency of the photon with which it is entangled to within the bandwidth of the originating pump photon (for biphotons produced via a spontaneous parametric downconversion process).